1. Field of the Invention
The present invention relates to a solid state image pickup element having a color filter for picking up a color image, and to a method of manufacturing a solid state image pickup element.
2. Related Background Art
FIG. 8 shows one of the structures, which have been known, as solid state image pickup elements, for use in color video cameras and color still cameras.
FIG. 8 is a side sectional view showing the structure of a conventional solid state image pickup element.
The solid state image pickup element shown in FIG. 8 has, in the vicinity of a surface of a semiconductor substrate 100, a photoelectric conversion portion 110 which generates signal charges according to the amount of incident light. The photoelectric conversion portion 110 is provided in each of pixels that are arranged to form a lattice pattern.
The pixels each have a driver circuit for generating a pixel signal in accordance with a signal charge that is generated in the photoelectric conversion portion 110, and for sending the pixel signal to a horizontal scanning circuit (not shown in the drawing) in response to a control signal sent from a vertical scanning circuit (not shown in the drawing) that is provided on an edge of the solid state photoelectric conversion element.
As shown in FIG. 8, gate electrodes 109 of transistors that constitute the above driver circuit are formed on the semiconductor substrate 100. On the gate electrodes 109, wiring layers are formed to suit the circuit structure of the driver circuit. In the structural example shown in FIG. 8, two wiring layers 105 and 107 are formed. Diffusion layers (not shown) which serve as sources and drains of the transistors are formed on portions of the semiconductor substrate that are under the gate electrodes.
Wires patterned to have a desired shape are formed in the wiring layers using aluminum (Al) or the like. Interlayer insulating films 106 and 108 formed from, for example, SiO2, are interposed between the wiring layers for insulation.
A first planarization film 104 is formed from, for example, acrylic resin, on the topmost wiring layer 105, covering the wires of the wiring layer 105. Provided on the first planarization film 104 are color filters 103 for splitting incident light in accordance with the color of a pixel. The color filters 103 are classified into three types by their colors (three primary colors of light): red (R) color filters formed from photo resist that contains a red pigment, green (G) color filters formed from photo resist that contains a green pigment, and blue (B) color filters formed from photo resist that contains a blue pigment. Each pixel is covered with one of the three types of color filters.
On the color filters 103, a light-transmissive second planarization film 102 is formed and microlenses 101 which are condensers for collecting incident light in the photoelectric conversion portion 110 are formed on the second planarization film 102.
FIGS. 9A to 9D are side sectional views illustrating a procedure of forming the conventional color filters that are shown in FIG. 8.
To form the color filters shown in FIG. 8, first, negative color resist with, for example, a green (G) pigment dispersed therein is applied to a surface of the first planarization film 104. Then a mask which blocks light except where green color filters (hereinafter referred to as G filters) are to be formed is placed for irradiation with ultraviolet rays having a wavelength of, for example, 365 nm, and for subsequent development. Thus G filters 103G are formed on the first planarization film as shown in FIG. 9A. The pattern of the G filters 103G measures 9.0 μm at the bottom portion (on the side of the first planarization film 104).
Next, as shown in FIG. 9B, negative color resist 103A with a red (R) pigment dispersed therein is applied to the entire top surface of the first planarization film 104 while covering the G filters 103G. Then a mask which blocks light except where red color filters (hereinafter referred to as R filters) are to be formed is placed for irradiation with ultraviolet rays having a wavelength of, for example, 365 nm.
At this point, in a conventional method of manufacturing a solid state image pickup element, a mask having a transmissive portion with a width of 9.0 μm, which is equal to the width of a region surrounded by the G filters 103G, is used in the ultraviolet irradiation (see FIG. 9B) and subsequent development to form an R filter 103R. In this case, the R filter 103R rises at its edges where the R filter meets the G filters 103G as shown in FIG. 9C. Furthermore, as the ultraviolet irradiation energy is increased, the edges of the R filter 103R climb over the edges of the G filters 103G as shown in FIG. 9D. In some conventional solid state image pickup element manufacturing methods, a mask having a transmissive portion that is slightly narrower than the width of the region surrounded by the G filters is used in ultraviolet irradiation to form the R filter 103R.
Details about how other components than the color filters are manufactured are omitted from the description here since they are irrelevant to characteristics of the present invention. Any known manufacture methods can be used to form other components than color filters as long as the optimal configuration is obtained for each of the other components.
In a solid state image pickup element having a large chip size, an image pickup region including the above vertical scanning circuit and horizontal scanning circuit is larger than the range that can be exposed to light in one shot of an exposure device (field size). Therefore, a split exposure method is employed in which the image pickup region is divided into plural exposure regions and the patterns obtained by the division are patched together to form a desired image pickup region. This method is described in JP 05-6849 A, for example.
Recent solid state image pickup elements suffer from various kinds of production fluctuation (such as mask size differences, microlens distortion components, exposure light distribution in one shot, mask alignment precision, and changes with time of resist characteristics) due to the increased level of pixel integration. Such production fluctuation causes fluctuation in size and shape of color filters or mispositioning of color filters, with the result that adjacent color filters overlap each other or a void is created between adjacent color filters.
In the solid state image pickup element manufacturing method described above, corners of the color filters are rounded because of lowering of the resolution of the exposure device, and a void where no color filters are formed is also created in such corners. Adjacent color filters overlap each other also when the energy of ultraviolet irradiation used in exposure of color resist is larger than necessary as described above and when the distance between adjacent color filters actually formed is shorter than designed. An example of eliminating such color filter overlapping and filterless void is disclosed in JP 10-209410 A. According to the method disclosed in this publication, a void between adjacent color filters is eliminated by forming three types of color filters on top of one another and then removing the upper layer color filters until the lowermost color filter is exposed.
As mentioned above, photolithography is usually employed in manufacturing color filters of solid state image pickup elements. In recent solid state image pickup elements where the pixel width is on the order of a few micron meters, a reduction projection exposure device called a stepper is also used to expose patterns of color filters to light in order to accurately form such small pixels.
If there is fluctuation in production process or mispositioning of a mask in forming color filters, the size and position of the color filters are varied depending on the degree of fluctuation. As a result, the shape and size of the color filters, the width of a void between color filters, and the color filter overlapping amount are varied from one exposure shot to another. Even when a mask having a transmissive portion that is narrower than the width of a region surrounded by G filters is used in the above ultraviolet irradiation to form an R filter, the size and position of the R filter are varied due to fluctuation in production process and mispositioning of masks. Accordingly, the width of a void between color filters and the color filter overlapping amount change so as to form distributions throughout the exposure shot plane, or are varied from one exposure shot to another.
A common solid state image pickup element has a chip size smaller than the field size of an exposure device and therefore a color filter pattern is formed in one exposure operation. In this case, if there are a void between color filters and overlapping of color filters as described above, the void width and the overlapping amount change continuously and two-dimensionally throughout the chip plane. Therefore, the amount of light that enters the photoelectric conversion portion changes in a manner that forms a two-dimensional distribution in the image pickup region. This means that a pixel signal outputted contains a shading component and a color mixture component corresponding to the void width and the overlapping amount, thereby causing degradation of a picked-up image.
In a solid state image pickup element formed by the above-described split exposure method, the size and position of color filters are varied from one exposure region obtained by the division to another exposure region. Therefore, it is very difficult to give color filters of different exposure regions the same size as well as to align a color filter of one exposure region identically to a color filter of another exposure region.
Light from an image pickup object enters a microlens formed on a surface of a solid state image pickup element through a light path indicated by a or b in FIG. 10, for example. Here, a ray a is transmitted through the microlens, a first planarization layer, a second planarization layer, a color filter, and several interlayer insulating films to reach a photoelectric conversion portion. On the other hand, a ray b enters from the interface between pixels and, after being scattered, reflected, refracted, diffracted and the like in the interface between color filters and in wiring layers, some components of the ray b reach the photoelectric conversion portion.
When there is a void in the interface between color filters, the absolute amount of incident light is about three times larger than when the interface has no void. Accordingly, if the void width varies, the amount of light that reaches the photoelectric conversion portion is varied by scattering, reflection, refraction, diffraction and the like. Furthermore, overlapping of adjacent color filters and a void between adjacent color filters give the second planarization film formed on the color filters insufficient levelness, and surface irregularities are created in the second planarization film above the color filter overlapping portion and the color filter void portion.
For instance, a concave portion in the second planarization film above the color filter void portion acts like a lens to disperse incident light as shown in FIG. 11A. As a result, dispersed light reaches the photoelectric conversion portion through color filters and causes fluctuation in amount of incident light among pixels.
In the color filter overlapping portion, on the other hand, light passes through the combined thicknesses of two color filters and is attenuated in the process before reaching the photoelectric conversion portion, resulting in fluctuation in amount of incident light among pixels. In addition, color is mixed as light passes through the color filter overlapping portion since adjacent color filters usually have different colors, and light of mixed colors reaches the photoelectric conversion portion. Furthermore, a convex portion formed in the second planarization film above the color filter overlapping portion as shown in FIG. 11B causes a change in amount of light that enters the photoelectric conversion portion indirectly through refraction of light. Therefore, despite equal conditions under which light from an image pickup object enters pixels, the absolute amount of light that reaches the photoelectric conversion portion varies from one pixel to another.
When color filters are formed by the above-described split exposure method, the color filter overlapping amount and the void width are varied among exposure regions divided as shown in FIG. 12 or FIG. 13 (a first exposure region and a second exposure region). This fact is reflected on difference in pixel signal voltage value (light detection sensitivity), which is determined by the amount of incident light, between the exposure regions and the difference is erroneously recognized as a difference in brightness of the picked-up image. As a result, when a solid state image pickup element formed by the split exposure method is used in picking up an image, visible streaks of image non-uniformity are found in portions of the picked-up image that correspond to interfaces between divided exposure regions.
Overlapping of color filters and a void between color filters are caused mainly by the exposure process as described above. Therefore, the overlapping amount and the void width can be reduced by various measures such as raising the precision of the size of masks used in exposure to light, correcting mispositioning of masks, correcting the exposure energy difference between exposure regions, and uniformizing characteristics of resist for forming color filters. However, it is impossible to reduce process fluctuation in an exposure region, namely, in-plane distribution, to nil. In addition, exposure process differences between exposure regions cannot be eradicated as long as the split exposure method above is employed. It has therefore been difficult in prior art to restrain within a certain range the difference in light detection sensitivity between exposure regions.